Contact is made with radiation-emitting optoelectronic components generally by applying a metallic connection contact to the semiconductor chip which contains the active zone. That surface of the semiconductor chip which is provided for contact-making is often simultaneously provided for coupling out radiation, so that the connection contact, which is not transparent to the emitted radiation, can be applied only to a partial region of the surface.
Particularly in the production of electrical contacts on p-doped semiconductor layers of III-V nitride compound semiconductors, the problem arises in this case that the semiconductor layer adjoining the connection contact has such a high resistance that the current flow through the active zone is essentially effected only through the regions directly beneath the connection contact. This adversely affects the brightness and the efficiency of the optoelectronic component.
In order to obtain a low forward voltage and a uniform high brightness over the entire chip area, a largely homogenous current density over the chip area is desirable. In order to obtain a lateral current density distribution that is as homogeneous as possible, U.S. Pat. No. 5,233,204, for example, discloses inserting between the connection contact and the underlying semiconductor layer a current expansion layer that is as thick as possible and has good conductivity, said current expansion layer being transparent to the emitted radiation.
Current expansion layers of this type should ensure an ohmic contact with the semiconductor and also be transparent and stable in respect of temperature and aging. Furthermore, the application of such a current expansion layer should be able to be integrated into the production process of the optoelectronic component in a simple manner. These multiple requirements considerably restrict the material selection for current expansion layers of this type.
A homogeneous current density over the chip area can be achieved in particular by means of a large-area current expansion layer which reaches as far as the sidewalls of the semiconductor chip. In this case, however, there is the risk of voltage flashovers occurring at the sidewalls of the semiconductor chip on account of electrostatic discharges (ESD). This risk exists particularly in the case of radiation-emitting semiconductor chips based on III-V nitride compound semiconductors since comparatively high internal electric fields occur in the case of the latter.